This invention relates to the fabrication of a vitreous bonded cubic boron nitride abrasive article wherein said article's core comprises silicon carbide and alumina and said article's rim comprises cubic boron nitride and alumina, said core and rim having substantially equal coefficients of thermal expansion to avoid generation of internal stresses at the boundary between said rim and core during the article's cooling after being vitrified at temperatures up to about 1900.degree. F.
It is well known in the art to use cubic boron nitride as an abrasive in grinding wheels. The cubic boron nitride preferred in the art has a zinc blend cubic structure with a resulting hardness approaching that of diamond and is described in U.S. Pat. No. 2,947,617 issued Aug. 2, 1960 to Wentorf. Cubic boron nitride in the form of an abrasive grain referred to as "borazon," is manufactured by the General Electric Co. and is relatively expensive. Notwithstanding its high cost, cubic boron nitride has been found to be useful in the grinding of metals and other hard materials. Thus, cubic boron nitride has been incorporated into metal bonded, organic resin bonded, and vitreous bonded grinding wheels.
In the grinding of metals and other hard materials, such as metal carbides, it is highly important that the grinding wheel be strong, resist thermal shock, resist mechanical shock, exhibit low wear, retain its shape, resist being loaded up by the material being ground (be free cutting), have good grinding efficiency and exhibit good metal removal rates. All of these attributes are of particular importance in a grinding operation such as the internal grinding of metal parts. It is, for example, particularly important that an internal grinding operation, such as the grinding of a bore in a metal part, that the grinding wheel maintain its shape and original dimensions for extended periods while exhibiting good strength, good grinding efficiency, and good metal removal rate. Excessive or uneven wear of the grinding wheel causes out of tolerance dimensions and undesirable alteration of the shape of the work piece.
These requirements also exist in the contour grinding of metals with preshaped grinding wheels, which requires that the wheel retain its initial shape and dimensions for long periods while having good grinding efficiency and metal removal rate. Similarly, these desirable attributes apply to all types of grinding to varying degrees. For instance, critical retention of grinding wheel shape is not as important as in other types of grinding, however, all grinding wheels must exhibit attributes which render them efficient and economical in use.
The coefficient of thermal expansion for preshaped grinding wheels used in either interior grinding operations or contour grinding operations must be known so that the grinding article can be intentionally undersized in a cold state, and thereby expand to the appropriate predetermined size from heat generated during use. Generally, the coefficient of thermal expansion for a grinding wheel consisting of several intimately mixed phases is related to the coefficient of expansion, weight fraction, bulk modulus, and density of each phase. The theoretical relation between these criteria and the average coefficient of thermal expansion is set forth in P. S. Turner, "Thermal Expansion Stresses in Reinforced Plastics," J. Research Natl Bur. Standards, 37[4]239-50 (1946); RP 1745.
One method in the prior art to incorporate bonded cubic boron nitride in a grinding wheel is to use an organic resin. However, these wheels are unsatisfactory in strenuous, high precision grinding operations, such as internal or contour grinding, because they readily lose their shape and have poor resistance to the high temperatures often encountered under strenuous grinding conditions. Examples of resin bonded cubic boron nitride grinding articles are described in U.S. Pat. No. 3,576,610 issued Apr. 27, 1971 to Mathewson.
Likewise, metal bonded cubic boron nitride grinding wheels have been fabricated, but are expensive, and consequently, their use has been confined principally to grinding very hard materials such as metal carbide cutting tool elements. Further, metal bonded cubic boron nitride wheels have a high coefficient of thermal expansion, and thus, their size and dimensions tend to change during use at varying loads. Metal bonded wheels also have the undesirable quality of loading up with the material being ground, and generally exhibit poor cutting rates. Examples of metal bonded cubic boron nitride articles are disclosed in U.S. Pat. No. 3,852,049 issued Dec. 3, 1974 to Hibbs, et al., which teaches the fabrication of a vitreous nitride product having a metal filler.
In the past, vitreous bonded cubic boron nitride grinding wheels have had limited success in commercial metal grinding operations, demonstrating superior usefulness where the grinding wheel is subjected to high mechanical and thermal shock, and is required to maintain its shape and dimensions over extended periods under strenuous grinding conditions. Typically, these articles are formed by cold pressing a mixture of cubic boron nitride, silicon carbide, and bonding medium to form the desired article, and then vitrifying said article at elevated temperatures up to about 1800.degree. F. to form the final product. The bonding medium is chosen to have a coefficient of thermal expansion substantially identical with the cubic boron nitride component to facilitate formation of the vitrified product. However, bond failure with a loss of the cubic boron nitride abrasive grain is the principal cause for the poor performance of vitreous bonded wheels under high thermal and mechanical shock grinding conditions. Additionally, vitreous bonded cubic boron nitride grinding wheels exhibit low grinding efficiency for many types of metal and often require relatively high grinding pressure or force to achieve grinding action thereby aggrevating the problems associated with bond failure.
Examples of compositions and methods for producing vitreous bonded cubic boron nitride abrasive articles are disclosed in U.S. Pat. No. 3,986,847, issued Oct. 19, 1976 to Balson, teaching a method for producing a vitreous bonded grinding wheel having a substantially uniform distribution of cubic boron nitride throughout the article. While fabricated from expensive materials, Balson's wheel is free grinding, can grind at low grinding pressures, has good adhesion between the bond material and the cubic boron nitride so as to resist rapid or premature breaking out of the cubic boron nitride grain with subsequent rapid wear of the wheel and loss of its shape, is resistant to mechanical and thermal shock, and has good grinding efficiency.
In light of the relatively high expense of cubic boron nitride attempts have been made in the prior art to concentrate boron nitride in a wheel's grinding surface or rim. For instance, attempts have been made to attach cubic boron nitride material to the outer surface of a grinding wheel with an epoxy. This method has been less than satisfactory because of poor adhesion at high grinding pressures and the resulting high temperatures generated during use. Examples of these methods are disclosed in U.S. Pat. No. 4,385,907 issued May 31, 1983 to Tomita, et al.
Similarly, attempts have been made to form vitreous grinding articles with a higher concentration of cubic boron nitride at the article's grinding surface than in its core. Attempts to fabricate such articles have been unsuccessful in light of the substantially different coefficients of thermal expansion of cubic boron nitride and other materials used in the article's cores, such as silicon carbide, alumina, quartz, and other bonding mediums. Because of differing coefficients of thermal expansion the rim and core sections of these vitreous articles typically separate during the cooling phase of production, or if separation is not pronounced, have high internal stresses generated at the interface between the rim and core components so that separation or cracking results during use.
Consequently, a need exists for an efficient grinding wheel having a core made of less expensive materials and a continuous rim of vitreous bonded cubic boron nitride. A need also exists for cost effective grinding article having differing rim and core compositions which is not subject to separation or stresses at the interface between the rim and core. Further, a need exists for a cost effective wheel utilizing inexpensive material for its core, while providing all of the advantages of vitreous cubic boron nitride grinding articles such as resistance to mechanical and thermal shock, ability to be free grinding, and good metal removal rates.
Yet a further need exists for a cold pressed product incorporating a rim and core as one body with subsequent firing providing a complete vitreous bonded body composed of two separate compositions. A further need exists for a vitreous glassy bonding medium that is compatible with cubic boron nitride to avoid rapid or premature breaking out of the boron nitride grain, and similarly compatible with a lower cost core composition to provide a totally integrated vitreous bonded article.